I set up the tripod in the back of the car so two legs were extended into the foot well, so they were firmly on the floor behind my front seats. Then I lifted the third leg (i have the ability on my tripod to lift the leg higher than the other two), and extended that so it was pressing on the back of the back seats, pushing the front legs into the back of the front seats.

I hope this makes sense!

Then I drove down the smoothest, straight road with dim orange street lighting I could find. I only went up to 25mph to keep the car as smooth as possible and I went down a straight road so I could keep my arms still.